Carbonaceous fabric laminate

ABSTRACT

A lightweight refractory thermal insulator is provided. Superposed layers of carbonaceous fabric are cemented together to form a laminate which, because of its low weight, structural integrity, and excellent thermal resistance, is useful as thermal insulation in a variety of applications. If desired, a thin uniform film or coating of carbon black may be interspersed between the layers of carbonaceous fabric in order to impart improved thermal properties to the laminate.

KR BwFM- HBTT United States Patent n91 Wessentiorf et al.

[ 1 Oct. 29, 1974 CARBONACEOUS FABRIC LAMINATE lnventors: Theodore RalphWesseodorl,

Florence, Ky.; John Michael Criscione, Broadview Heights, Ohio UnionCarbide Corporation, New York, N .Y.

Filed: Dec. 6, 1971 Appl. No.: 205,407

Related 05. Application Data Continuation of Ser. No. 846,252, July 30,1969, abandoned.

Assignee:

lnt. Cl 1332b 11/10 Field of Search 161/182, 156; 252/502; 23/209.1

References Cited UNITED STATES PATENTS 9/1956 Horvitz 204/67 3,l07,l$2\'l0ll963 Ford et a1. 3,174,895 31965 Gibson Primary Examiner -George F.Lesmes Assistant Examiner-Patricia C. lves Attorney, Agent, or Firm-JohnS. Piscitello ABSTRACT A lightweight refractory thermal insulator isprovided. superposed layers of carbonaceous fabric are cemcnted togetherto form a laminate which, because of its low weight, structuralintegrity, and excellent ther- 8 Claims, No Drawings 114.. sot-1,

I CARBONACEOUS FABRIC LAMINATE CROSS REFERENCE TO RELATED APPLICATION-This application is a continuation of application Ser. No. 846,252,filed July 30, 1969, now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to refractory materials for use as thermal insulation and moreparticularly to laminated carbonaceous articles comprising superposedlayers of carbonaceous fabric.

2. Description of the Prior Art The continuing and acceleratingtechnological ad vance brought about by research on materials for use inspace vehicles has resulted in the development of a large number ofmaterials. Despite all this research, however. the materials developedto date as backings for reentry heat shields in space vehicles have notproven entirely satisfactory in that they are either too heavy for theirintended purpose, have poor structural integrity, or have been found notto possess maximum insulating properties under the wide variety ofconditions in which space vehicles operate. Consequently, efforts havecontinued in the search for an improved heat shield backing. Such amaterial should 'be lightweight, possess good structural integrity. andact as a thermal insulator over a wide range of temperatures. Inaddition to heat shield applications, a material having such propertieswould find wide use in a variety of applications where such combinationof properties is desirable.

The primary object of this invention, therefore, is to provide anarticle with good structural properties which is an excellent thermalinsulator overa wide temperature range and yet is lightweight and of alow density so that it may be readily employed as a heat shield backingand in other applications.

SUMMARY OF THE INVENTION Broadly, the object of the invention isaccomplished tacting relation in a non woven fibrous body. Air layingoperations such as carding and garnetting which effect a relativelyoriented disposition of fibers into a felted sheet are suitable for thispurpose. When a more ran- I dom deposition of fibers is desired, such asin the production of battings, conventional textile devices which effectthe air laying of fibers in a random webbing can be employed.

Felt is the preferred fabric for use in the invention. Most preferably,the felt is prepared by water laying short carbon or graphite fibersusing conventional paper making techniques. The paper-thin felt sheetswhich can be prepared in this manner are the most preferred form offabric for use in preparing the laminated articles of this invention.

When preparing felt or paper" by the water laying I of carbon orgraphite fibers, the fibers are first cut or by providing a laminatestructure formed by bonding superposed sheets of a carbonaceous fabricwhich may be either a nonwoven carbon or graphite fibrous material suchas felt or batting, or a woven fabric such as carbon or graphite cloth.The term carbonaceous" as used throughout this specification is intendedto include both the graphitic and non-graphitic forms of carbon.

DESCRIPTION OF THE PREFERRED EMBODIMENTS graphite fibers by any method,either wetor dry, which effects the disposition of such fibers inintimately conchopped to a size suitable for processing, e.g., aboutone-fourth inch in length; homogeneously intermixed with water and asuitable binder, such as starch or other well known binder, to form anaqueous slurry; and then deposited from the slurry on a substrate toform a sheet. This sheet is then processed by conventional paper makingtechniques to produce the final carbonaceous product.

Converting the fiber slurry into sheets of felt or paper involves threegeneral steps, or modifications of these, by which all commercial-basepapers are made:

I. The arrangement of the fibers in the slurry into a wet sheet;

2. The removal of a portion of the free water from the wet sheet by wetpressing this is reflected by improved physical characteristics of thepaper;

3. The progressive removal of additional water by heat. In principle, awet sheet is generally formed either by running a dilute suspension offibers evenly onto the surface of a moving endless belt of wire cloth,through which excess water may be drained, or by running an endless beltof wire cloth through a suspension of fibets. In the first case theFourdrinier process a part of the water drains off by gravity, a part istaken from the sheet by suction, and a part is removed by pressure; inthe second case. a vacuum is maintained below the stock level in thecylinder in which the wire cloth is rotating and the sheet forms on thewire by suction much as does a cake on a vacuum filter. Most papergrades are formed by the first process; very lightweight tissues andmany grades of paperboard are made by the second. In either case, thethickness of the sheet is controlled by the speed of travel of themachine, by the consistency (ratio of fiber to water) of the suspensionor by the amount of stock allowed to flow onto the machine.

Woven carbonaceous fabrics such as carbon and graphite cloth are alsosuitable for use in the instant invention. These materials are availablecommercially and are generally produced by the techniques described inUS. Pat. Nos. 3.01 l,98l, 3,107,152 and 3,116,675.

After the carbonaceous fabric sheets have been prepared they areassembled in the desired configuration in a laminated assembly. Bestresults are achieved employing carbonaceous paper" from 0.006 to 0.012inches thick. Greater or lesser thickness dimensions,

however, will also provide excellent results.

Prior to being assembled in a laminate, the sheets of carbonaceousfabric are lightly coated with a carbonizable resin in an amountsufficient to securely bond them together. The carbonizable resins whichcan be used are any binders or cements commonly used for bonding carbonor graphite and include. among others. coal tar pitches, phenolics.epoxies, furanes and the like. In order to insure an even distributionof resin on the carbonaceous fabric, the resin is preferably dissolvedin a suitable solvent and the carbonaceous fabric is soaked in thesolution. The carbonaceous fabric is then removed from the solution andthe solvent evaporated, leaving a uniform coating of resin on thefabric. If desired, the sheets may then be coated with a thin uniformlayer of carbon black flour.

Once assembled, the laminate is placed in a press and a suitablecompressive pressure, e.g., from about 500 psi. to about i500 psi.. isapplied while the press platen temperature is raised, if necessary, to atemperature sufficiently elevated to cure the resin. Heating iscontinued until the resin is cured. The assembly is then baked to effectcarbonization of the resin, e.g., at a temperature of from about 500C.to about 200C, preferably from about 700C. to about l000C. When thelaminate is prepared from the preferred fonn of carbonaceous fabricsheets, i.e., from carbonaceous paper" having a thickness of 0.006 to0.012 inches, and compressive pressures in the amount stated above areapplied to form the laminate, the carbonaceous paper" employed isgenerally reduced in thickness to from 0.00l to 0.003 inches as a resultof the pressure applied.

Any inert liquid solvent capable of dissolving the carbonizable resinemployed and vaporizable at a temperature lower than that at which theresin reacts (i.e., the temperature at which the resin cures orcarbonizes) can be employed in preparing the laminate structures of theinstant invention. Generally, the carbonizable resin is present in thesolution in an amount of from about 5 per cent by weight to about 75 percent by weight, preferably from about per cent by weight to about 25 percent by weight. Suitable solvents include, among others. saturatedaliphatic hydrocarbons such as hexane. heptane, pentane. Lsooctane.purified kerosene. and the like; saturated cycloaliphatic hydrocarbonssuch as cyclopentane; cyelohexane, methylcyclopentane,dimethylcyclopentane, and the like; aromatic hydrocarbons such asbenzene, toluene. xylene. and the like; and ketones such as acetone, andthe like.

if desired, after the solvent has been evaporated from the carbonaceousfabric sheets, the sheets may be "dusted" with a thin uniform layer ofcarbon black flour prior to being assembled in the desired laminateconfiguration and baked. This results in a laminate structure havinglayers of carbon black interspersed between the carbonaceous fabricsheets. The carbon black film disrupts the contact between the sheets ofcarbonaceous fabric and thereby provides a more effective thermalbarrier. For this reason, laminate structures wherein the carbonaceousfabric sheets have been dusted with carbon bhck are the preferredembodiment of the invention.

Any form of carbon black. e.g., gas blacks, furnace combustion blacks,furnace thermal blacks. lampblacks, may be employed to dust the resincoated sheets of carbonaceous fabric. The cmbon black flour ispreferably applied to the surface of the carbonaceous sheets to athickness of less than 0.001 inch, but can be applied in greaterthicknesses. e.g., from 0.001 inch to about 0.002 inch.

The carbon black flour may be applied to the resin coated carbonaceousfabric sheets in any suitable manner, e.g., by suspending the carbonblack in a suitable gaseous vehicle and spraying the mixture on thesubstrate to the desired thickness, e.g., by means of a conventional airgun. Air is the preferred gas because it is inexpensive and readilyavailable, but any inert gas which will not react with the carbon blackparticles, carbonaceous fabric sheets, and resin binder employed canalso be used, e.g., inert gases such as nitrogen, carbon dioxide, argon,krypton, xenon, and the like, are suitable.

The laminates prepared by dusting the resin coated carbonaceous fabricsheets with carbon black generally contain, after carbonization, fromabout 3 per cent by weight to about 40 per cent by weight, preferablyfrom 1 about 10 per cent by weight to about 25 per cent by weight, ofcarbon black; from about 15 per cent by weight to about 67 per cent byweight, preferably from about 35 per cent by weight to about 55 per centby weight, of carbonaceous fabric; and from about 30 per cent by weightto about 45 per cent by weight, preferably from about 35 per cent byweight to about 40 per cent by weight, of carbonized binder.

The laminates which have not been dusted with carbon black generallycontain, after carbonization, from about 25 per cent by weight to aboutper cent by weight, preferably from about 65 per cent by weight to about75 per cent by weight, of carbonaceous fabric; and from about 20 percent by weight to about 75 per cent by weight, preferably from about 25per cent by weight to about 35 per cent by weight, of carbonized binder.

The laminate structures of the instant invention may be prepared invarious sizes and shapes. Thus, for example, flat plate laminates up to0.04 inches thick have been prepared as well as frustrum shaped bodies20 inches long with a major diameter of 8 inches and a 6 degree halfangle. Composites have also been laid up in a concentric layer patternand in an inter-leaf ply pattern.

ln order to test the effectiveness of the laminate structures of theinvention as thermal insulators, a number of laminates were fabricatedand tested for thermal diffusivity and thermal conductivity. Thus,carbon paper laminates were fabricated from 4 X 4 X 0.0l0 inches sheetsof carbon paper. The sheets of carbon paper were immersed in a solutionof acetone containing 20 weight per cent of a phenolic resin of thenovolac type together with a hardening agent therefor, allowed to standuntil the acetone evaporated, and then assembled into a laminatestructure by stacking !0 sheets in a parallel fashion on top of oneanother. A sheet of aluminum foil was placed on the top and bottom ofthe stack and the assembly was placed in a press and a compressivepressure of 1000] psi. was applied while the press platen temperaturewas raised to C. to cure the resin. Heating was continued for about 2hours. The laminate was then placed between two graphite plates, packedin coke, and heated to 800C. at a rate of 5 l0C./hour to carbonize theresin.

The thermal diffusivity, thermal conductivity and short beam shearstrengths of a number of laminates so prepared are listed in Table 1below along with the val- Ja /AN... 0.00"... s

' ues obtained for a laminate containing ten sheets of TABLE l In orderto measure the effect of hightemperature on the laminate structures ofthe invention. a number of composites were prepared in the mannerdescribed above and tested for thermal diffusivity and thermalconductivity at elevated temperatures. The data obtained is shown inTable 2. It is apparent from Table 2 that the laminates of the inventionare not substantially affected by being subjected to high temperaturessuch as 2000C. As shown therein. thermal conductivity increases slightlyat higher temperatures. but still is significantly low. particularly inview of the low density which is maintained throughout the temperaturecycle.

Room Temperature Thennal Properties and Short Beam Shear Strengths ofLaminate Insulators Notes: The samples were evaluated by the laser pulsetechnique.

"' 4/I SPHII-UPdCPTh, measured parallel to laminate. The carbon "paper"of the laminates after compression was from 0.00) to 0.003 inches thick.Sample (I) contained ten sheets of carbon "paper" bonded together with33 weight percent of carbonized resm timed on the weight of the entirecomposite). Sample (2) contained ten sheets of carbon paper bondedtogether with 38 weight percent of carbonized resin (based on the weightof the entire composite). Sample (3) contained ten sheets of carbon"paper" interspersed wit h ni ne layers of carbon black, each layer ofcarbon black having a thickness of less Than 0. 1 inches, with the totalweight of the carbon biack being percent of the total weight of theentire composite and the total weight of the carbonized resin kingpercent of the total weight of the entire composite.

' TABLE 2 High Temperature Thermal Properties" of Laminate Insulators DcT l C th nsity emmraturfi llfitgrma on uctivity Sample" lnrtral ma A E.l er. t usivrty BT% ft (glcc) (glee) l ('C) (cmlsec) t r- (I) Carbonpaper laminate 0.7! 0.52 2025 H 0.0050 0.3l 0.65 I995 970 0.0057 0.450.75 I795 670 0.006I 0.56 (2) Carbon "paper laminate 0.8! 0.73 I670 7600.0039 0.34 I960 940 0.0050 0.45 (3) Carbon "paper" carbon blacklaminate 0.93 1520 I20 0.0022 0.24

0.89 2020 540 0.0027 0.32 (4) Carbon "paper" carbon black lamittl: 0.67I358 495 0.0033 0.25 I595 6I0 0.004l 0.3l 0.64 2005 l 770 0.0022 0."0.64 i978 976 0.0037 0.29

Notes:

" Thermal properties of Samples l and 2 were determined using the arcimage 360' cyclic phase shift method in argon atmosphere.

Thermal properties of Sample 3 were determined using the arc image Hi0argon atmosphere. The first three determinations of thermal pro cyclicphase shift method with quadratic correction in rties of Sample 4 (atl358C.. l$C.. and 2005C. avenge e temperatures. respectivel were madeusing the are ima e 180 cyclic phase shilt method. with quadraticcorrection in argon atmosphere being made or the third determination.The average temperature) was made rairtg the arc image 360'c average oftwo sample determinations.

ourth determination of thermal properties of Sample 4 (at I978 yclicphase shift method in argon atmosphere. Each value represents the Thecarbon "paper of the laminates after compression was from 0.001 to 0.003inches thick. Sample I contained ten sheets of carbon "paper" bondedtogether with 30 weight rcent of carbonized resin (based on the weightof the entire composite) while Sample 2 contained ten sheets of carbonpaper" inded together with 38 weight percent of carbonized resin (basedon the weight of the entire composite). Sample 3 contained ten sheetsofcar of carbon black having a thickness of less than 0.00l inches boopaper interspersed with nine layers of carbon black. each layer with thetotal weight of the carbon blaclt being 20 percent of the total weightof the entire composite 44kt total weightof the carbonized resin being30% of the total weight of the entire composite. Sample 4 contained tensheets of carbon "paper interspersed with nine layers of carbon black.each layer of carbon black avittg a thickness uflcss than 0.00l tracks.with the total Wcigtt of carbon black being 5 percent of the totalweight of the entire composite and the total weight of the carbonizedresin being 30 of the total weight of the entire composite.

Final density measured after exposure to maximum temperature.

Temperature average of front and back faces of laminate.

' Temperature diiference between heels andfront faces.

l. A laminate structure having a thennal conductivity no greater than0.077 BTU-ft./ft.-hr.-F. and a thermal diffusivity no greater than0.0024 cmF/second at room temperature, as measured by the laser pulsetechnique. comprising a plurality of superposed layers of nonwovenpaper-thin felt made from carbon fibers, wherein each layer ofpaper-thin felt is from 0.001 to 0.003 inches thick, bonded togetherwith a carbonized binder.

2. A laminate structure as in claim I in which the carbon fibers of thepaper-thin felt layers are about onefourth inch in length.

3. A laminate structure as in claim 1 having from five to layers ofpaper-thin felt.

What is claimed is:

l5 tolSla ersot a 4. A laminate structure as in claim 3 m which the caryp bon fibers of the paper-thin felt layers are about 1/4 inch in length.

5. A laminate structure having a thermal conductivity no greater than0.077 BTU-tt.lft.-hr.-F. and a thermal diffusivity no greater than0.0024 cm./second at room temperature, as measured by the laser pulsetechnique, comprising a plurality of superposed layers of nonwovenpaper-thin felt made from carbon fibers. wherein each layer ofpaper-thin felt is from 0.00] to 0.003 inches thick and is interspersedwith thin uniform layers of carbon black, each layer of carbon blackbeing less than 0.001 inches thick, and bonded together with acarbonizable binder.

6. laminate structure as in claim Sin which the carbon fibers of thepaper-thin felt layers are about l/4 inch in length.

7. Klamihate structure as in claim 5 having from five er-thin felt.

8. A laminatestructure as in claim 7 in which the carbon' fibers of thepaper-thin felt layers are about onefourth inch in length.

# t i t i

1. A LAMINATE STRUCTURE HAVING A THERMAL CONDUCTIVITY NO GREATER THAN0.077 BTU-FT./FT,2-HR.-*F. AND A THERMAL DIFFUSIVITY NO GREATER THAN0.0024CM,2/SECOND AT ROOM TEMPERATURE, AS MEASURED BY THE LASER PULSETECHNIQUE, COMPRISING A PLURALITY OF SUPERPOSED LAYERS OF NON-WOVENPAPER-THIN FELT MADE FROM CARBON FIBERS, WHEREIN EACH LAYER OFPAPER-THIN FELT IS FROM 0.001 TO 0.003 INCHES THICK, BONDED TOGETHERWITH A CARBONIZED BINDER.
 2. A laminate structure as in claim 1 in whichthe carbon fibers of the paper-thin felt layers are about one-fourthinch in length.
 3. A laminate structure as in claim 1 having from fiveto 15 layers of paper-thin felt.
 4. A laminate structure as in claim 3in which the carbon fibers of the paper-thin felt layers are about 1/4inch in length.
 5. A laminate structure having a thermal conductivity nogreater than 0.077 BTU-ft./ft.2-hr.-*F. and a thermal diffusivity nogreater than 0.0024 cm.2/second at room temperature, as measured by thelaser pulse technique, comprising a plurality of superposed layers ofnon-woven paper-thin felt made from carbon fibers, wherein each layer ofpaper-thin felt is from 0.001 to 0.003 inches thick and is interspersedwith thin uniform layers of carbon black, each layer of carbon blackbeing less than 0.001 inches thick, and bonded together with acarbonizable binder.
 6. A laminate structure as in claim 5 in which thecarbon fibers of the paper-thin felt layers are about 1/4 inch inlength.
 7. A laminate structure as in claim 5 having from five to 15layers of paper-thin felt.
 8. A laminate structure as in claim 7 inwhich the carbon fibers of the paper-thin felt layers are aboutone-fourth inch in length.